It prob won’t have one and even if it did it would be perished now. So yes a Dpm sandwich will sort it out.
Give the floor a scape with a blade scraper 1st.
Then na straight to it.
You only need to do one coat of liquid Dpm.
Fill the hole in with sand and cement on a visqueen sheet lapping up the doorframe.
Get it close the height of the main floor.
Let that set.